Okay, this is actually really simple. All you need is the mole/mole ratio (which you have in the equation) and the molar mass of each substance, which you get from a periodic table. Find the mass of N2O5, which is 108.01 g/mol, and HNO3, which is 63.01. Set up the equation \[0.65g N _{2}O _{5}^{?}(\frac{ 1 mol N _{2} O _{5}}{ 108.01 g })(\frac{ 2 mol HNO _{3} }{ 1 mol N _{2} O _{5} })(\frac{ 63.01 g HNO _{3} }{ 1 mol HNO _{3}}) = \] and solve.